In the mining industry, high voltage, polyphase A.C. power is supplied through a relatively long cable from an appropriate source to a load that is frequently mobile. The cable includes a plurality of power conductors, usually one for each phase, as well as a plurality of ground conductors, usually a number equal to the number of phases. The majority of cables have the power and ground conductors symmetrically arranged. All of the ground conductors are connected together at the two ends of the cable, with the common connection at the supply source being connected to a neutral terminal through a resistor for safety purposes.
The ground conductors of the cable are susceptible to breaking, causing open circuits which result in an unbalanced voltage being induced in equipment at the load end of the cable. The induced unbalanced voltage is a potential safety hazard to personnel working around the equipment. The extent of the safety hazard is realized by the enactment of a United States Federal Law requiring the ground condition to be monitored by passing a signal from an insulated ground conductor to a frame of equipment at the load end of the cable. The signal must be returned through the ground conductor to a safety ground at the source.
Devices to monitor the condition of the ground conductor have been introduced into the market place. Generally, these devices can be divided into four different categories as follows:
1. Sensing reflected impedance between the ground conductor and a pilot line; PA1 2. Sensing the impedance between the ground conductor and a pilot line; and PA1 3. Transmitting a D.C. or A.C. signal from the load end of the cable through the ground conductors to a full-wave bridge rectifier at the source end of the cable. PA1 4. Applying a D.C. signal to a loop which includes the ground conductors and measuring the potential developed across a shunt placed in the ground conductors.
Each of these devices has its own advantages and disadvantages regarding sensitivity, cost, safety, probability of malfunctioning, effects of transients and cable length, as well as induced A.C. voltages in the cable which are potentially dangerous. Also, the use of a pilot wire in many of the systems potentially requires the replacement of the existing cables, which do not have such a wire therein.